1. Field of the Invention
The present invention relates to a bias current supply circuit and an amplification circuit including the bias current supply circuit. Particularly the invention relates to the high-efficiency, high-power amplification circuit which is configured to use a bipolar transistor and operated by low power-supply voltage, and the bias current supply circuit which is added to the amplification circuit.
2. Related Background Art
In the high-efficiency, high-power amplification circuit which is configured to use the bipolar transistor and operated by the low power-supply voltage, since a collector current is greatly influenced by a fluctuation in temperature, a base bias current is supplied by a current mirror circuit which is configured to use a diode-connected bipolar transistor.
FIG. 1 shows a circuit diagram of a first example of the conventional current mirror type base bias current supply circuits.
The current mirror circuit shown in FIG. 1 is the current mirror type base bias current supply circuit having the simplest configuration. The current mirror circuit includes a resistor R and a diode-connected bipolar transistor Q which are connected in series between a control potential node to which control voltage Vcon is supplied and a ground potential node, and the base bias current is supplied from a connection node OUT between the resistor R and a collector of the transistor Q.
The high-efficiency amplifier having a wide output dynamic range and linearity of gain can be realized by setting a bias condition to class B to cut passage of idle current. However, in actual fact, since strain caused by a fluctuation in gain is increased by non-linearity of mutual conductance of an element, the linearity of the gain is maintained in the wide output dynamic range by setting the bias condition to class AB in which the idle current passes through the amplifier to a certain extent.
In the class AB amplification circuit configured to use the bipolar transistor, since average collector current is increased according to an increase in output level, it is necessary that a bias circuit sufficiently supplies an increased amount in average base current according to the increase in average collector current. However, in the current mirror circuit including the diode-connected bipolar transistor shown in FIG. 1, the sufficient current can not be supplied.
Therefore, the current mirror circuit which supplies the base current through an emitter follower circuit is widely used in order to decrease output impedance.
FIG. 2 shows the circuit diagram of a second example of the conventional current mirror type base bias current supply circuits.
The conventional current mirror type base bias current supply circuit shown in FIG. 2 includes a resistor R1 and diode-connected bipolar transistors Q2 and Q1 which are sequentially connected in series between the control potential node to which the control voltage Vcon is supplied and the ground potential node, a bipolar transistor Q3 in which the collector is connected to a power supply potential node to which power supply voltage Vcc is supplied and a base is connected to a collector of the transistor Q2, and a resistor R2 which is connected between an emitter of the transistor Q3 and the ground potential node. The base bias current is supplied from the connection node OUT between the emitter of the transistor Q3 and the resistor R2.
However, since the conventional current mirror type base bias current supply circuit shown in FIG. 2 has the configuration in which the bipolar transistors are connected in two-stage series, unless the control voltage Vcon is sufficiently increased more than double voltage of on-voltage Vbeon of the transistor, it is impossible to maintain compensation for a fluctuation in bias current to temperature change, in which responsiveness is required.
However, in a system such as a cellular phone in which the control voltage is low, that the control voltage Vcon is increased is contrary to a flow of technical advance, so that there is generated a problem. Particularly, in the system in which linear operation is required in the wide output dynamic range like a CDMA communication system, a fluctuation in idle current caused by the temperature change, which is largely affected during low output, becomes the problem.
For the purpose of a countermeasure against the above problem, a complex type bias current supply circuit in which the transistor turned on by one-stage on-voltage Vbeon is added has been proposed.
FIG. 3 shows the circuit diagram of a third example of the conventional current mirror type base bias current supply circuits.
The conventional current mirror type base bias current supply circuit shown in FIG. 3 includes a resistor R1 and diode-connected bipolar transistors Q2 and Q1 which are sequentially connected in series between the control potential node to which the control voltage Vcon is supplied and the ground potential node, a bipolar transistor Q4 in which the collector is connected to the power supply potential node to which the power supply voltage Vcc is supplied and the base is connected to the collector of the transistor Q2, a resistor R2 which is connected between the control potential node and the emitter of the transistor Q4, and a diode-connected bipolar transistor Q3 which is connected between the emitter of the transistor Q4 and the ground potential node. The base bias current is supplied from the connection node between the emitter of the transistor Q4 and the collector of the transistor Q3.
The base bias current is supplied to the base of a bipolar transistor RFTr for RF signal amplification through a choke inductor L. The transistor RFTr is connected between the power supply potential node and the ground potential node, an input RF signal RFin is inputted to the base of the transistor RFTr through a capacitor C, and an output RF signal RFout is outputted from the collector of the transistor RFTr.
FIG. 4 is a graph showing temperature characteristics of the collector bias current of the bipolar transistor for RF signal amplification to the control voltage Vcon in the case where the conventional current mirror type base bias current supply circuit shown in FIG. 3 is used. Specifically, the graphs T1, T2, and T3 show the temperature characteristics at ambient temperatures 90° C., 30° C., and −30° C. respectively.
In the case where the conventional current mirror type base bias current supply circuit shown in FIG. 3 is used, assuming that the control voltage Vcon is set to, e.g. the voltage as low as 2.8V, the collector bias currents of the bipolar transistor for RF signal amplification become 27 mA, 35 mA, and 45 mA at the ambient temperatures −30° C., 30° C., and 90° C. respectively. This fluctuation range is decreased and improved, compared with the case in which the conventional current mirror type base bias current supply circuit shown in FIG. 2 is used.
However, in the case where the conventional current mirror type base bias current supply circuit shown in FIG. 3 is used, in fact the fluctuation range is not sufficiently decreased, because it is necessary that the sufficient current passes through the transistor Q4 in order to decrease the output impedance.
Further, since the RF signal is leaked into the bias current supply circuit due to the low output impedance of the bias current supply circuit, as shown in FIG. 3, the choke inductor L arranged between the output node of the bias current supply circuit and the bipolar transistor RFTr for RF signal amplification is essential for prevention of the RF signal.